Automatic vehicle location systems

ABSTRACT

An automatic vehicle location center includes a control center and a plurality of mobile units. Each mobile unit is able to derive a current position from external radio signals. A prediction is responsive to this current position signal and to predicted motion of the mobile unit to derive a predicted position signal. The predicted and current positions signals are compared and the current position signal is transmitted to the control center in dependence on the result of the comparison. The control center has a predictor, similar to that in each mobile unit, responsive to the current position signal and to predicted motion of the mobile unit to derive a predicted position. This position data is only transmitted from a mobile unit to the control center if the predicted position signal differs significantly from the actual position.

FIELD OF THE INVENTION

This invention relates to automatic vehicle location systems of the typewhich may be used, for example, to monitor the locations of each of aplurality of mobile units such as a fleet of vehicles.

BACKGROUND OF THE INVENTION

A wide range of different basic techniques exist for automatic vehiclelocation. These include satellite based location systems such as, forexample, the Omega system, and also systems which use local radiobeacons. A mobile unit operating in one of these systems monitors phasedifferences in signals received from different satellites and uses theseto derive the current position of the mobile unit. Typically apositional accuracy of approximately 100 meters may be obtained for asingle derivation of position.

In a system comprising a central controller and a fleet of mobile unitsthe positional information for each unit is required at the controllerso that decisions relating to a mobile unit may be made, e.g. joballocation for taxis. In existing systems the information is transmittedto the controller by polling each mobile unit in the system for itsposition at regular intervals. In current systems the most frequentpolling interval possible is generally fixed and is often as infrequentas once every 50 seconds, i.e. each mobile unit derives its position andtransmits this information to the controller every 50 seconds.

Using such a polling system introduces an error in the data available atthe controller and this error will vary with time elapsed since the lastpolling of a mobile unit for an update of its positional data. Forexample, a vehicle cruising at 120 kilometers per hour will travel 1666meters in 50 seconds. Thus the average positional error at thecontroller will be slightly over 800 meters. Even if a vehicle is stuckin traffic at 25 kilometers per hour it will travel 347 meters in 50seconds thus having an average positional error over the 50 secondperiod of 173.5 meters. This error is, of course, in addition to theerror in the originally derived position of the vehicle. Systems whichhave errors of this magnitude are not able to make best use of mobileunits when allocating them to particular jobs.

Another disadvantage of systems which poll mobile units for theirpositions on a periodic basis is the amount of radio data communicationstraffic generated. This traffic can easily occupy most of thetransmission time on a single radio channel and thus, if a significantnumber of vehicles are to receive data from the controller, more thanone radio channel is required. Using current technology a single mobileradio channel might normally support approximately 100 vehicles withoutany vehicle location system in operation. A further two radio channelsmight very well be needed to carry the positional updating informationfrom a vehicle location system provided in each mobile units if regularupdates were to be produced once every 50 seconds.

SUMMARY OF THE INVENTION

Preferred embodiments of the present invention provide a system forautomatic vehicle location which greatly increases the accuracy ofpositional information available by significantly reducing the errors inpositional information which are inherent in a conventional polledvehicle location system. At the same time the amount of radio datacommunications traffic is significantly reduced.

This is achieved by providing both at the controller and at each mobileunit a predictive coder responsive to a position signal derived at thevehicle and to the vehicle's velocity (speed and direction) at the timethat position signal was derived to derive a predicted position for themobile unit. Thus the controller and each mobile unit have a predictedcurrent position signal available to them.

At each mobile unit the predicted position signal derived by thepredictive coder is compared with an actual position signal derived fromthe vehicle location unit. If the difference between the actual positionsignal and the predicted position signal exceeds a predetermined limitthen the latest actual position signal and velocity are transmitted tothe controller and used as a new seed input for the predictive coder. Atthe same time this latest actual position signal and velocity are alsoused as a new seed input to the predictive coder at the mobile unit.

The invention is defined in its various aspects in the appended claimsto which reference should now be made.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the invention will now be described in detail, by wayof example, with reference to the single figure which shows a blockdiagram of circuitry provided at the controller and at a mobile unit.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the figure a controller 2 is shown which comprises atransmitter/receiver 4 coupled to an antenna 6. The transmitter/receiveris coupled to a microprocessor 8. Signals for mobile units are generatedby the microprocessor 8 and are sent to the units by thetransmitter/receiver 4 and the antenna 6. Signals from the mobile unitsare received, by the antenna 6 and the transmitter/receiver 4 and arethen supplied to the microprocessor 8.

The microprocessor 8 is also coupled to a predictive coder 10. Thisreceives a position and a velocity signal from the microprocessor and,using these, and a linear prediction technique supplies repeatedlyupdated predicted position signals to the microprocessor based on theoriginally supplied position and velocity signals and time elapsed sincea mobile unit was last known to be at a particular position.

A mobile unit 12 comprises an antenna 14 coupled to a transmitterreceiver 16 which is in turn coupled to a microprocessor 18. Data issent to and received from the controller 2 via the transmitter receiver16 and the antenna 14.

The mobile unit also comprises a location system responsive to signalsfrom local beacons or from satellites or from any other form of locationsystem to derive the position of the mobile unit. This is typicallyderived to within an accuracy of the order of 30 to 300 meters. Thelocation system 20 repeatedly calculates the position of the mobile unitand supplies it to the microprocessors 18. The location system 20updates the position signal sufficiently quickly for the microprocessorto, effectively, continuously monitor the position.

The microprocessor 18 is also supplied with a velocity signal derivedfrom the speed and direction of the mobile unit.

A second predictive coder 22 is coupled to the microprocessor 18 andthis is equivalent to the predictive coder 10 at the controller. Thuswhen fed with a position and a velocity signal the predictive coder 22can derive a predicted position for the mobile unit based on a elapsedtime since the position signal was generated.

In operation the mobile unit 12 initially derives its position using thelocation system 20. This is supplied to the microprocessor 18 whichtransmits it and the velocity signal of the mobile unit to thecontroller 2 via the transmitter receiver 16 and the antenna 14. At thecontrol center the position signal and the velocity signal are fed tothe predictive coder 10. At the same time the velocity signal andposition signal are fed to the predictive coder 22 in the mobile unit12. The predictive coders 10 and 22 then supply to their respectivemicroprocessors 8 and 18 predicted position signals for the mobile unitbased on elapsed time since the position and the velocity signals weregenerated. Thus at the controller position data is available based onthe last known position and known velocity of a mobile unit.

At the mobile unit the microprocessor 18 continuously monitors thepredicted position signal derived by the predictive coder and comparesit with the current position signal derived by the location system 20.Providing the vehicle continues to move at an average velocity close tothat initially fed to the predictive coder there will be littledifference between the predicted position signal and the actual positionsignal. If, however, the mobile unit's velocity changes and it deviatesfrom the path derived by the predictive coder the actual position andthe predicted position will rapidly diverge. When the difference betweenthe predicted position and actual position exceeds a predeterminedamount the microprocessor 18 will cause a new actual position signal andvelocity signal to be transmitted to the controller 2 to act as newseeds for the predictive coder 10. This updated position and velocitywill also be fed to the predictive coder 22 at the mobile unit as newseeds. The same process will then continue.

Using such a system positional updates are sent from a mobile unit tothe controller only when the mobile unit has deviated from the predictedpath by more than a predetermined amount. In all other circumstances nopositional update needs to be transmitted to the controller which, ofcourse, has available to it a predicted position on which it may baseany decisions relating to that mobile unit.

The update message sent from the mobile unit 12 to the controller 2 willtypically be coded into a packet of data of, for example, 24 usercharacters. The nature of mobile data communication systems is such thatfor short messages the message duration is dominated by irreduceibleoverheads and thus very little more radio air time is needed to send amessage of 24 user characters than is needed to send a message purelycontaining location data. For this reason it is possible to send in thedata packet data relating to the position, velocity, time to which theposition data refers and, if desired, data relating to rates of changeof speed or of direction. Using this additional data more complexpredictive coding algorithms can be used to derive the predictedposition of a mobile unit more accurately.

Because data messages are only sent from mobile units to the controller2 when the difference between predicted and actual positions exceeds apredetermined amount the amount of radio traffic between mobile unitsand the control center is considerably reduced. For example, a parkedvehicle will not send any update to the control center since its actualposition and its predicted position will remain the same. A vehiclecruising on a motorway at a steady speed will only occasionally deviatefrom its predicted position by an amount in excess of the predeterminedthreshold (tolerable communications error) and will thus onlyoccasionally require updated positional information to be sent to thecontroller.

By appropriately programming the microprocessor 18 in a mobile unit theunit can adapt the tolerable communications error in dependence on theactual location of the mobile unit. For example, when the mobile unit iswithin a dense city center a low threshold is required, for example, 50to 100 meters. In less dense areas, e.g. rural areas, it is notnecessary to have such an accurate estimation of the position of amobile unit and a threshold of, for example, 300 to 500 meters would beappropriate.

Thus it can be seen that a vehicle location system which uses the systemdescribed above is able to operate with a reduced amount of radiotraffic compared to a system which polls mobile units for theirlocations and furthermore the control center has available to it a moreaccurate estimation of the location of a mobile unit than withconventional polled systems.

I claim:
 1. An automatic vehicle location system comprising a controlcenter and a plurality of mobile units, each mobile unit comprisingmeans for deriving a current position signal from external radiosignals, prediction means responsive to a previously derived currentposition signal and to predicted motion of the mobile unit to derive apredicted position signal, means for comparing the current positionsignal with the predicted position signal and means for transmitting apositional update and a velocity signal to the control center independence on the result of the comparison, the control centercomprising prediction means responsive to the positional update and thevelocity signal to derive a predicted position signal for that mobileunit.
 2. An automatic vehicle location system according to claim 1 inwhich each prediction means derives predicted motion for a mobile unitfrom the velocity signal and from time elapsed since derivation of thepositional update signal last sent to the control center.
 3. Anautomatic vehicle location system according to claim 2 in which eachmobile unit further includes means for deriving a rate of change ofspeed signal from the motion of the mobile unit, said positional updateincludes the rate of change of speed signal, and the prediction means isalso responsive to the rate of change of speed signal included in thepositional update.
 4. An automatic vehicle location system according toclaim 2 in which each mobile unit further includes means for deriving arate of change of direction signal, said positional update includes therate of change of directional signal, and each prediction means is alsoresponsive to the rate of change of direction signal.
 5. An automaticvehicle location system according to claim 2 in which the positionalupdate and velocity signal are transmitted from a mobile unit to thecontrol center when the result of the comparison is that the differencebetween current and predicted signals exceeds a predetermined threshold.6. An automatic vehicle location system according to claim 5 includingmeans for altering the threshold.
 7. An automatic vehicle locationsystem according to claim 1 in which the control center prediction meansis responsive to current position signals from and to predicted motionof each of the plurality of mobile units to derive a predicted positionsignal for each mobile unit.
 8. An automatic vehicle location systemaccording to claim 1 in which the positional update is a 24 characterdata packet.
 9. An automatic vehicle location system according to claim3 in which each mobile unit further includes means for deriving a rateof change of speed signal from the motion of the mobile unit, saidpositional update includes the rate of change of speed signal, and theprediction means is also responsive to the rate of change of speedsignal included in the positional update.
 10. An automatic vehiclelocation system according to claim 3 in which each mobile unit furtherincludes means for deriving a rate of change of direction signal, saidpositional update included the rate of change of directional signal, andeach prediction means is also responsive to rate of change of directionsignal.
 11. An automatic vehicle location system comprising a controlcenter and a plurality of mobile units, each mobile unit comprisingmeans for deriving a current position signal from external radiosignals, prediction means responsive to a previously derived currentposition signal and to predicted motion of the mobile unit to derive apredicted position signal, means for comparing the current positionsignal with the predicted position signal and means for transmitting apositional update and a velocity signal to the control center independence on the result of the comparison, the control centercomprising prediction means responsive to the positional update and thevelocity signal to derive a predicted position signal for that mobileunit, in which each prediction means derives predicted motion for amobile unit from the velocity signal and from time elapsed sincederivation of the positional update signal last sent to the controlcenter, and in which the positional update and velocity signal aretransmitted from a mobile unit to the control center when the result ofthe comparison is that the difference between current and predictedsignals exceeds a predetermined threshold;said system further includingmeans for altering the threshold, in which the threshold altering meansis responsive to the current position signal to automatically alter thethreshold.
 12. A mobile unit for use in an automatic vehicle locationsystem comprising means for deriving a current position signal,prediction means responsive to a previously derived current positionsignal and to a velocity signal from the mobile unit to derive apredicted postion signal for the mobile unit, means for comparing thecurrent position signal with the predicted position signal, and meansfor transmitting a positional update and a velocity signal to a controlcenter in dependence on the result of the comparison.